Primary mineral structures

The proof of reversibility in primary mineral weathering would be instances where primary mineral structures have formed under earth-surface conditions. There are reports that secondary quartz can slowly precipitate at room temperature from solutions supersaturated with monosilicic acid. More typically, however, precipitated silica in soils is structurally disordered, in the form of chalcedony or opal. In fact, as long as alumina is present, silica does not precipitate as a separate phase, reacting instead to form aluminosilicates (layer silicates, imogolite, or allophane). 

Silica minerals are a primary mineral classified as tectosilicates, characterized by repeating SiO units in a framelike stracture. Quartz, one of the most abundant minerals on earth, often comprises up to 95% of aU sand and silt fractions. It therefore is representative of the structure and properties of sihca minerals. 

Models for the formation of Precambrian sediments suggest that the chemical sediments (such as cherts) of the Isua supracrustal belt have formed as shallow water deposits. This is in agreement with structures locally preserved in the metacherts of the sequence. After deposition, the supracrustals were folded and metamorphosed. Finally, the metamorphism reached lower amphibolite facies and in consequence, most of the primary minerals became recrystallized. As a result all chert now appears as quartzite. But apparently metacherts, magnetite iron formation and quartz carbonate rocks have retained their major element chemistry largely unaltered during metamorphism (Nutman et al., 1984) 119). 

The inorganic minerals of soil are classified into (a) primary minerals and (b) secondary minerals (Table 3.3). Primary minerals are minerals with the chemical composition and structure obtained during the crystallization process of molten lava, whereas secondary minerals are those that have been altered from the original structure and chemical composition by weathering, a process referred to as the geomorphic cycle (Fig. 3.2). Generally, the size of soil mineral particles varies from clay-sized colloids (< 2 pm) to gravel (< 2 mm) and rocks. 

Primary Mineral Classification Based on Structural Arrangement... 

The biocompatible CBPC development has occurred only in the last few years, and the recent trend has been to evaluate them as biocompatible ceramics. After all, biological systems form bone and dentine at room temperature, and it is natural to expect that biocompatible ceramics should also be formed at ambient temperature, preferably in a biological environment when placed in a body as a paste. CBPCs allow such placement. We have discussed such calcium phosphate-based cements in Chapter 13. Calcium-based CBPCs, especially those constituting hydroxyapatite (HAP), are a natural choice. HAP is a primary mineral in bone [3], and hence calcium phosphate cements can mimic natural bone. Some of these ceramics with tailored composition and microstructure are already in use, yet there is ample room for improvement. This Chapter focuses on the most recent biocompatible CBPCs and their testing in a biological environment. To understand biocompatible material and its biological environment, it is first necessary to understand the structure of bone and how it is formed. 

The most noticeable isotopic difference between saline waters from crystalline rocks and sedimentary formation waters is their position above the meteoric waterline. This is postulated to be due to mineral hydration reactions in a very water-depleted environment (Fritz and Frape, 1982). Several recent smdies have suggested that hydration reactions in low water to rock environments can occur and result in increasing salinity. The incorporation of OH into primary silicate such as amphiboles and phyUosUicates (where OH crystal lattice sites are part of the mineral structure) is suggested as one mechanism for controlling solute concentration (KuUemd, 2000). The formation of secondary OH containing mineral phases such as zeolites and clays can also continue to consume water molecules and concentrate the residual fluids both chemically... 

In Figure 2.2, the relationship between soil particle size and mineralogy is illustrated qualitatively, demonstrating the prevalence of secondary minerals in the clay fraction. The primary minerals, which originally formed under conditions of high temperature and pressure, are unstable in the soil environment. Consequently, primary minerals, when they are reduced to small particle size by physical weathering, tend to chemically decompose (weather) rapidly. Nevertheless, clay-sized quartz particles persist in soils because of the resistance of the quartz structure to chemical decomposition, as will be discussed later in this chapter, but even this mineral eventually dissolves as silica is leached from the soil. Thus we see that reaction rates... 

The principal primary mineral groups are the silica minerals (including quartz), feldspars, feldspathoids, olivines, pyroxenes, amphiboles, and micas. All are silicates and can be classified structurally based on the arrangement of connected silica (Si04) tet-rahedra. Table 2.2 summarizes the classification system for these common rockforming minerals. 

Layer Silicates. Although the common primary minerals include island, chain, sheet, and framework silicates, the most stable and persistent silicates, which occur as weathering products (secondary minerals) in the clay fraction of soils, are sheet silicates. Figure 2.9a depicts the structure of the tetrahedral sheet in these minerals, which is comparable to the tetrahedral structure of mica. For the layer silicate clays, however, numerous structural combinations of the tetrahedral sheet with octahe-drally coordinated metal cations are possible. 

The primary mineral, chlorite, which occurs in rocks as large crystals, possesses an interlayer sheet composed largely of Mg(OH)2. Since the mineral brucite is composed of magnesia sheets with the same basic structure, the single interlayer sheet in chlorite is termed the brucite layer. Isomorphous substitution of part of the Mg by produces a positively chained hydroxide sheet (see formula above) that props the 2 1 layers apart at a c-spacing of 14 A. This rigid interlayer contrasts with the hydrated interlayer of vermiculite, and even though chlorite and vermiculite have similar c-... 

The transition and heavy metals, referred to hereafter as trace metals, are important to plants and animals as both micronutrients and toxic elements. Many of them occur in the soil environment in cation form. As naturally occurring elements, some of these cations are incorporated into primary and secondary mineral structures and may be very unavailable. Schemes for complete extraction of these metals from soils require extreme treatments, including dissolution of certain minerals. As pollutants, the metals may enter the soil in organically complexed form or as metal salts. In the latter case, the metal cations then adsorb on mineral and organic surfaces. 

Silica, alumina, iron, and the various base cations that are dissolved by primary mineral weathering can precipitate as new low-temperature minerals—a process known as neoformation. This should be distinguished from weathering processes, termed alteration, in which part of the parent mineral structure is inherited by the weathering product. Examples of alteration are given in the previous section. 

Chlorites in soil occur as primary minerals derived from mafic rocks and as secondary minerals from the weathering of biotite, hornblende, and other amphiboles and minerals (Bamhisel, 1977). Chlorites are 2 1 1 minerals consisting of 2 1 mica structure in addition to an interlayer hydroxide sheet. Chlorites have low CEC and surface areas. 

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